BTW for future people - this was because NM ported from dbus-glib to GDBus between wily and xenial, and dbus-glib has a misfeature where it will return the value if you miscapitalise the first letter of a property, instead of erroring. GDBus doesn't do that, so the misspelling of 'State' as 'state' was correctly causing a check in the wireless page to fail, which took an error path that made Ubiquity not load the page.
BTW for future people - this was because NM ported from dbus-glib to GDBus between wily and xenial, and dbus-glib has a misfeature where it will return the value if you miscapitalise the first letter of a property, instead of erroring. GDBus doesn't do that, so the misspelling of 'State' as 'state' was correctly causing a check in the wireless page to fail, which took an error path that made Ubiquity not load the page.